Dear Authors,

Thanks for preparing your thorough revisions in response to the first round of reviewer comments. I am going to pass your manuscript and responses back to reviewers now, but I have a few of my own comments that I would like you to consider addressing at this stage. I have a set of high-level comments, but also some line comments as well.

High-level comments:

1. I suggest further distinguishing the paper’s contribution in the text. I think I am still struggling to discern this paper’s core contribution. I don’t mean to suggest it’s not a contribution, only that I think further clarifying it could help readers. As far as I can tell, the paper does not deliver any protocol, and it also does not really deliver any results. So, is its purpose ultimately to motivate the value of having the groundwater sector in isimip, and describe some of the models and variables that will be looked at, but without detailed protocols or results given? If so, I would suggest stating this much more explicitly. Also, regarding results, I do like have your section is titled to clarify its purpose “4. Unstructured experiments point out model differences that should be explored further”. But earlier in the abstract you refer to ‘initial results’, which I suggest modifying because I think it could lead to misinterpretation about what this paper actually provides. I do actually agree with reviewer one’s original comment that the paper might be better off spending less time showing results from a range of papers/experiments whose points of commonality are hard to discern (section 4), and instead spending more time on the paper’s core contribution to introduce the new isimip-groundwater sector, but I respect that you wish to keep this section, and you already provided rationale in your response to reviewers about why you feel it is important to keep.
2. Consistency with ‘global change’ terminology and literature. The paper’s title and exposition invoke ‘global change’, but I find that this paper is more aligned with climate change and global hydrology, rather than global change. It is true that some of the key models referenced in your study represent dynamics around land use change, climate change scenarios, and human groundwater abstraction. So, in this sense your study is theoretically aligned with global change, assuming you ultimately conduct experiments that explore these dimensions. But, what I think has the potential to create confusion is that there are many researchers who focus centrally on ‘global change’, and many of them study or consider water resources in various ways, usually focused on human dimensions, economics, etc. I think it will be unclear to those communities how your contribution aligns (or does not) with their efforts, and it could be helpful clarify this in the paper. For example, as I mention in my line comments below, there are researchers who work on groundwater and global change through a hydro-economics lens, focused foremost on the economics of groundwater extraction, but also including some physics-based representations of groundwater extraction. Many of those teams are also directly linked (and internally consistent) with the IAMs that produce the emissions and land use change trajectories (e.g., via ScenarioMIP) that are responsible for feeding climate impacts information into the hydrology models that you are focused on. How will the groundwater sector in ISIMIP relate to those communities and any global change scenarios they produce related to groundwater?
3. I don’t find that it’s possible for me to reproduce the figures/results you present in the paper. Please provide a code repository that complies with GMD standards.

Line comments (line numbers, when provided, pertain to the tracked changes PDF)

Abstract

• Line 35. I suggest avoiding normative language like “excessive”, and instead using more specific language (i.e., excessive in what way), if possible, to avoid misinterpretation.
• Line 38. “no ensemble of groundwater models has been available”. I think it would be helpful to articulate how you define a groundwater model for purposes of this paper. I assume you are referring to models focused largely on the physical system. But, for example, there are global hydro-economic models that deal explicitly with the economics of groundwater extraction along with some groundwater physics. Are those models excluded? I assume so, because they are not cited.
• The sudden discussion of Spain and Portugal in the abstract felt a bit surprising and arbitrary. Maybe rearranging your current text could soften some of the surprise.
• It is only at the very end of the abstract that we learn that this entire exercise is really focused on climate impacts. I personally think it could lead to misinterpretations (from those not very familiar with ISIMIP) to feature that only at the end of the abstract, particularly given the title's use of "global change".

Introduction

• “The pressure on groundwater systems intensifies due to the combined effects of population growth, socioeconomic development, agricultural intensification, and climate change, e.g., through a change in groundwater recharge (Taylor et al., 2013; Reinecke et al., 2021).” I find the cited literature to be insufficient. These papers may have referenced these issues or addressed a subset of these issues, but as far as I can tell, the list of papers does not cover the full space. It could be helpful to cite the papers that have addressed these issues holistically, including those that have looked at combined effects of multiple global change factors on groundwater.
• Line 71: (1) what is a “large-scale perspective”? (2) You are citing economics as a key issue, which again makes me concerned that your paper actually omits a lot of humans- and economics-focused global groundwater literature. (3) The cited literature is also non-exhaustive.
• Line 79: This statement seems like it could benefit from further literature support: “While large-scale climate-groundwater interactions are starting to become understood (Cuthbert et al., 79 2019), current global water and climate models may not always capture these feedbacks as most either do not consider groundwater at all or only include a simplified storage bucket, limiting our understanding of how climate change will affect the water cycle as a whole.”
• I suggest better explaining what the role of the global water sector in isimip is, so readers can better understand the full picture and the implications of a groundwater sector.

Sections 2/3: ISIMIP framework/current generation of models

• Here could be a place to consider describing what kinds of models are included versus excluded—like hydroeconomic models and global change models that are more focused on humans and less focused on groundwater physics.

Results

1. In Figure 4, I found it confusing that you say you’ll focus on green and orange—and then orange is called ‘focus’.

Vision

• You note that “In summary, the ISIMIP Groundwater sector aims to enhance our understanding of the impacts of climate change and direct human impacts on groundwater and a range of related sectors.” It’s certainly possible I missed it, but I don’t see that this is actually supported/made clear in the paper. What set of experiments will be run that will isolate the impacts of humans on groundwater? What dimensions of human impacts will be looked at, and how? I think this probably just relates to my earlier comment that I’m struggling with wanting more details about what experiments will be done, whereas doing so isn’t necessarily the focus of this paper.
• A lot of what you describe is indeed what other communities (including, but not limited, to other isimip sectors) have found about the value of producing ensembles that enable intercomparison. I suggest citing those other communities’ work.